Air-cooled Chiller Research Articles

A Novel Control Strategy for Air-Cooled Twin-Circuit Screw Chillers

Air-cooled chillers are generally the major electricity consumers in air-conditioned buildings in the subtropical climate. To improve the energy efficiency of the air-cooled chillers at part load conditions, two or more refrigeration circuits are designed. This paper considers how the use of optimal circuit loading sequence (CLS) enables these chillers to operate more efficiently. A thermodynamic model for the air-cooled chillers with twin refrigeration circuits was developed using TRNSYS and validated using a wide range of operating data. Based on the sophisticated chiller model, an analysis was carried out on how the chiller COP varied with different CLS and variable condensing temperature control (CTC). A chiller plant designed for a representative office building in Hong Kong was investigated to assess the potential electricity savings by use of CLS. 4.2% of the annual electricity consumption by chillers could be achieved with optimal CLS compared with the base case. Under CTC, the coefficient of performance (COP) was improved further, and CTC coupling with optimal CLS would reduce the annual total electricity consumption for cooling by 9.6%.

Hybrid Artificial Neural Network−Genetic Algorithm Technique for Condensing Temperature Control of Air-Cooled Chillers

Air-cooled chillers are commonly used in commercial buildings in the subtropical climate, which are considered inefficient due to operating under traditional head pressure control. This study presents a hybrid intelligent control technique, including neural networks and genetic algorithms, for the optimal control of the set points of the condensing temperature to improve the coefficient of performance (COP) of air-cooled chillers under various operating conditions. The neural network is used to model the air-cooled chillers, and genetic algorithm is adopted in searching optimal set points of condensing temperature based on the predicted fitness values. Results show that this control technique allows optimal set point of the condensing temperature to be successfully determined, and the chiller performance can be improved considerably.

Comparative study on the energy performance of chiller system in an institutional building with stochastic frontier analysis

This study demonstrates how to compare the energy performance of an existing air-cooled chiller with and without mist precooling of condenser air, considering different operating conditions between the two situations. A comprehensive set of operating data was gathered for each situation for over 4 months. Using stochastic frontier analysis, technical efficiency (TE) for the coefficient of performance (COP)—output—was evaluated for each set of operating conditions—inputs—involving weather and controlled operating variables. TE had an average of 0.64 and a standard deviation of 0.14 under mist precooling, while the corresponding values were 0.56 and 0.18 in the absence of mist precooling. A positive correlation was identified between the COP and TE. Based on the frequency distribution of TE, mist precooling enabled the chiller to attain higher performance more frequently under standardized operating conditions. Pearson correlations among the variables ascertain that TE has the highest correlation with condensing temperature, followed by the temperature of supply chilled water, so their precise control would give performance enhancement. TE can provide a basis of benchmarking chiller performance at different operating conditions and help explain the extent of improvement brought from the weather responsive control.

Experimental and numerical investigations of the energy efficiency of conventional air conditioning systems in cooling mode and comfort assurance in office buildings

This paper provides a comparative study of the energy efficiency of a conventional ventilating and air conditioning (VAC) system with three configurations. This system was used to cool an experimental room in an office building in more control scenarios realised by adopting different cooled water temperatures and chiller cooling systems. In this purpose, an air–water mist cooled system for an air-cooled chiller is proposed. Depending on the components, the analysed VAC system has the following configurations: (1) air handling unit (AHU) and fan-coil units (FCUs); (2) air handling unit (AHU); and (3) heat recovery unit (HRU) and fan-coil units (FCUs). The electricity consumption weight of the equipment of each VAC system, the total energy consumption and the influence of control parameters on this consumption are analysed. The results of the experiments indicate that the optimal efficiency VAC system in office buildings includes an AHU and FCUs with an air–water mist cooled chiller and a cooled water temperature of 8°C. For this system, a mathematical model is developed to minimise the electricity consumption by optimal control, where the consumption depends only on non-controlled parameters (solar radiation intensity and outdoor air temperature). This model is verified with actual measurements to confirm its accuracy, and a good agreement between the prediction and measurement is achieved. Additionally, the assessment of thermal comfort in the experimental office is performed based on the PMV-PPD model using the ASHRAE Thermal Comfort programme. The obtained results demonstrate that the AHU&FCUs system assures an increase in thermal comfort. Finally, a TRNSYS simulation model of energy consumption and PMV-PPD indices for the AHU&FCUs system is developed and experimentally validated.

액체식 제습시스템의 냉각수 공급온도에 따른 제습효율 변화 및 Water-Side 프리쿨링 적용성 분석

This research is an experimental study of the liquid desiccant system performance. In the absorber, dehumidification process and sensible cooling of outdoor air takes place at the same time. The dehumidification effectiveness variation with the chilled water temperature changes in the absorber based on the experiment data collected from the liquid desiccant system operation. The liquid desiccant system used in this research dehumidifies <TEX>$2000m^3/h$</TEX> process air and 24.5kW air-cooled chiller or 136.1kW cooling tower may provide chilled water at target temperature. Experiments were conducted on the day of peak cooling load of outdoor air in the summer season when the outdoor air was hot and humid(at Incheon, South Korea. Aug 2013). Based on the experiment data showing the dehumidification performance variation of the liquid desiccant system for the cooling water temperature, a validation of the dehumidification effectiveness values of a liquid desiccant system is compared with existing model.

SIMULATION ON OPERATION CHARACTERISTICS OF HYBRID GROUND-AIR SOURCE HEAT PUMP SYSTEM WITH NATURAL COLD STORAGE

To solve the problem of the coefficient of performance (COP) decrease and power crunch in cooling dominated South China associated with the long-term usage of ground-source heat pump system (GSHPS), a hybrid ground-air source heat pump system (HGASHPS) with natural cold storage is presented in this paper. The system consists of a GSHP system and compound air-cooled chillers (CACC), which can be operated according to the vapor compression refrigeration cycle or separate type heat pipe natural cycle. The mathematical models of each part of the system were set up and the conversion conditions between operation modes for the system were determined. The transient simulation for HGASHPS with natural cold storage in Nanjing was carried out. The operation characteristics of the systems in operational life were comparatively analyzed. The simulation results indicated that the HGASHPS with natural cold storage could retain the thermal balance of soil temperature field in one year cycle and increase the COP and reliability of the system.

Passive intensification of the ammonia absorption process with NH3/LiNO3 using carbon nanotubes and advanced surfaces in a tubular bubble absorber

The present study aims to quantify experimentally the individual and simultaneous effects of CNTs (carbon nanotubes) and advanced surfaces on the performance of an NH3/LiNO3 tubular bubble absorber. Operating conditions are those of interest for use in air-cooled absorption chillers driven by low temperature heat sources. Firstly, experimental tests were performed with the tubular absorber fitted with an inner smooth surface to analyze the effect of adding carbon nanotubes (0.01 wt%) to the base mixture NH3/LiNO3. Then, the tubular absorber was tested using an inner advanced surface tube both with and without adding carbon nanotubes to the base mixture NH3/LiNO3. The advanced surface tube is made of aluminum and has internal helical micro-fins measuring 0.3 mm in length.Results show that the maximum absorption mass flux achieved with the CNT binary nanofluid and the smooth tube is up to 1.64 and 1.48 times higher than reference values at cooling-water temperatures of 40 and 35 °C, respectively. It is also found that simultaneous use of CNT nanoparticles and advanced surfaces resulted in a more pronounced increase in the absorption mass flux and solution heat transfer coefficient with respect to the smooth tube absorber with NH3/LiNO3 as a working pair.

Performance evaluation of air-cooled screw chillers at low part load ratios and outdoor temperatures in Dubai and measures to improve the performance

Air-cooled chillers are considered to be intensive energy consumers and account for over one-fourth of the energy consumption in air-conditioned buildings. In Dubai, for about 4-5 months, chillers operate at low part load ratios with a reduced coefficient of performance (COP) due to low outdoor temperatures. This paper evaluates the performance of air-cooled twin-circuit screw chillers serving a medium-scale commercial building in the month of January when the outdoor temperatures are low. The performance of the chillers are analysed in view of calculated parameters and optimal conditions obtained from the readings taken. The chillers are found to be operating poorly at low COPs. Measures to rectify the problems in the performance of the chillers and advanced chiller technologies that are currently under research are discussed.

Review of Standards for Energy Performance of Chiller Systems Serving Commercial Buildings

A prescriptive approach is primarily used to establish energy benchmarks for energy systems in buildings. This paper reviews national standards and guidelines for the energy performance of chiller systems which provide cooling energy in commercial buildings but their operation causes the major proportion of electricity consumption in the building sector. The standards reviewed are currently adopted in nine places (in alphabetical order): Australia, California, Canada, China, Chinese Taipei, the EU, Hong Kong, New Zealand and the USA. Under standard rating conditions at full load operation, the minimum required coefficient of performance (COP) ranges from 2.40 to 3.06 for air-cooled chillers and from 3.80 to 6.39 for water-cooled chillers. Some standards state also the COP requirements at part load operation with integrated part load values or seasonal energy efficiency ratios. A classification scheme is provided in the EU and Chinese standards which helps labeling and certifying high efficient chiller products. Criteria for enhancing energy ratings of chiller systems are suggested.

Ground source heat pump performance in case of high humidity soil and yearly balanced heat transfer

Ground source heat pump (GSHP) systems are spreading also in Southern Europe, due to their high energy efficiency both in heating and in cooling mode. Moreover, they are particularly suitable in historical cities because of difficulties in the integration of heating/cooling systems into buildings subjected to historical preservation regulations. In these cases, GSHP systems, especially the ones provided with borehole heat exchangers, are a suitable solution instead of gas boilers, air-cooled chillers or cooling towers. In humid soils, GSHP systems are even more interesting because of their enhanced performance due to higher values of soil thermal conductivity and capacity. In this paper, GSHP systems operating under these boundary conditions are analyzed through a specific case study set in Venice and related to the restoration of an historical building. With this analysis the relevant influences of soil thermal conductivity and yearly balanced heat transfer in the design of the borehole field are shown. In particular, the paper shows the possibility to achieve higher compactness of the borehole field footprint area when yearly balanced heat transfer in the borehole field is expected.Then, the second set of results contained in the paper shows how GSHP systems designed for high humidity soils and yearly balanced heat loads at the ground side, even if characterized by a compact footprint area, may still ensure better performance than other available and more common technologies such as boilers, air-cooled chillers, chillers coupled with cooling towers and heat pumps and chillers coupled with lagoon water. As a consequence, in this last comparison, the efficiency of GSHP systems is confirmed even when compact borehole installation areas are used. The analysis is performed in terms of both input energy consumption ratio and primary energy ratio and takes into account the energy consumption of auxiliary devices as well. The conclusions are drawn in terms of percentage, thus allowing the reader to extend the results achieved in this paper to systems operating in other contexts under similar boundary conditions.

Energy and exergy analysis of absorption–compression hybrid air-conditioning system

A hybrid system is proposed based on a driving unit integrated with an air-cooled compression and an air-cooled absorption chiller. The cooling capacity of the hybrid system is used for air conditioning of a residential building. The prime mover can either be a micro-gas turbine, a gas turbine, or a solid oxide fuel cell. In order to investigate the effect of the operating conditions and the type of driving unit, energy and exergy analyses have been carried out. The efficiency factors and the exergy destruction in each component have also been studied. The results show that the maximum irreversibilities happen in the generator or in the air-cooled condenser, and the total destruction is higher for the gas turbine case compared to the other two cases. It appears that the increase in the ambient temperature causes a decrease in the energy utilization factor but an increase in the second law efficiency. This process is reversed with an increase in evaporating temperature, whereby an increase in the energy utilization factor and a decrease in the second law efficiency is experienced.

The Development of Dual-Effect Fuzzy Control System Based on FPGA

The water misting cooling has phase change characteristics,so the cooling efficiency is better than water cooled ,and more saveing energy、save water、less noise ,It is being widely used in the air conditioning system for the condenser cooling. In order to enhance the cooling efficiency of the water misting cooling system and improve the efficiency of the air conditioning system in the cold room, In the study use ultrasonic water misting cooling chiller for controlled object, and proposed the dual-effect fuzzy control system controller’s design concepts ,carry out the fuzzy controller and the control circuit in the FPGA chip to reach the goal of the product, After the relevant tests that is differing from traditional air-cooled water chillers. In the study, in addition to improve the shortcomings of traditional chiller energy consumption, to enhance the efficiency of water misting cooling system, achieve more saving energy and high efficiency control purposes.

Energy simulation of sustainable air-cooled chiller system for commercial buildings under climate change

Air-cooled chiller systems are commonly used to provide cooling energy in commercial buildings but with considerable electricity consumption. This study demonstrates how to operate the systems with sustainable performance under climate change by applying optimal condenser fan speed control coupled with mist pre-cooling of air entering the condensers. The simulation of system performance was carried out by using EnergyPlus together with an experimentally verified chiller model capable of analyzing advanced control strategies. Weather data under three climate change scenarios in 2020, 2050 and 2080 were considered in evaluating the cooling demand and hourly electricity consumption of an air-cooled chiller system serving an office building. It is found that optimal condenser fan speed control coupled with mist pre-cooling of air-cooled condensers can help maintain a higher coefficient of performance under the warmer future climate, reducing the annual electricity consumption by 16.96–18.58% in the reference weather year and 2080. The ways to implement the sustainable control have been discussed.

Letters from the edge: Less conventional acoustical solutions

The essence of acoustical analysis and problem solving still is source–path–receiver; yet, as standards, criteria and products evolve, solution precedents should be revalidated or invalidated, so that new, innovative approaches may be considered. While weighing the best interests of the client, should one rigidly follow procedures developed long ago for average or anticipated conditions, or might the conventional wisdom not always be correct? This paper presents a series of short case-studies where noise or vibration were treated at the source and/or along the path, but with some “twist” or variation, either from typical solution applications or owner/client response. Relevant standards, ordinances, or criteria are referenced. Where available, on-site acoustical measurements, observations, photos, or receiver experiences illustrate concepts. For the case studies presented: indicate (i) importance of the problem, (ii) method of the development used for problem solving, (iii) original contribution of the work, and (iv) conclusions. Case studies may include any of the following: standby power generators for data center, university cafeteria, and dining hall, coffee grinding and packaging shop within a grocery market, exercise/therapy floor impact above offices, air-cooled refrigeration chillers near residences, high-rise elevator equipment room adjacent to residential units, and/or high-rise domestic water booster/circulation pump room adjacent to residential unit.

Optimizing the pad thickness of evaporative air-cooled chiller for maximum energy saving

It is favorable to increase the energy efficiency of an air-cooled chiller with an efficient, reliable and cost-effective method. Coupling a direct evaporative cooler with an air-cooled condenser seems to be a feasible solution to this problem. To maximize the energy saving potential of an evaporative air-cooled chiller (EACC), which composes of an evaporative air cooler and a conventional air-cooled chiller, a mathematical model was developed and a new index, increase of seasonal energy efficiency ratio (ISEER), was proposed to evaluate the energy saving potential of the evaporative air-cooled chiller. The energy saving potential of EACC was simulated by using the hourly weather data for four typical cities in China. The impacts of various factors on the energy saving potential were analyzed and it was found that there exists an optimal pad thickness which maximizes the energy saving. The optimal pad thickness varies with climatic condition and face velocity. Optimization results of the pad thickness in 31 main cities in China are presented. The maximum energy saving potential of EACC in China is between 2.4% and 14.0% depending on the climatic condition. It is necessary to determine optimal values for the design parameters according to different climatic condition for maximum energy saving.

Improved Energy Performance of Air-Cooled Liquid Chillers with Innovative Condensing-Coil Configurations

The purpose of this study was to develop mathematical models for air-cooled chillers and their components using innovative varied row configurations as a parameter analysis model followed by a simulation of actual operational performance. In this manner, we were able to observe the increase in performance of air-cooled chillers and the energy transfer efficiency of individual components. This study found that the innovative varied row configuration (Type C) can increase the COP of air-cooled chiller by 6.7% over that of traditional condensing-coil configuration (Type A) with an increase in total irreversibility and the irreversibility of the condenser of 8.4% and 4.1%, respectively; Type C can increase the COP of air-cooled chiller by 3.3% over that of the best condensing-coil configuration (Type B) with an increase in total irreversibility and the irreversibility of the condenser of 1.6% and 4.6%, respectively. We believe that the results of this research can provide an important basis of reference for future design of air-cooled chiller units.

Energy-efficient HVAC systems: Simulation–empirical modelling and gradient optimization

Abstract This paper addresses the energy saving problem of air-cooled central cooling plant systems using the model-based gradient projection optimization method. Theoretical–empirical system models including mechanistic relations between components are developed for operating variables of the system. Experimental data are collected to model an actual air-cooled mini chiller equipped with a ducted fan-coil unit of an office building located in hot and dry climate conditions. Both inputs and outputs are known and measured from field monitoring in one summer month. The development and algorithm resulting from the gradient projection, implemented on a transient simulation software package, are incorporated to solve the minimization problem of energy consumption and predict the system's optimal set-points under transient conditions. The chilled water temperature, supply air temperature and refrigerant mass flow rate are calculated based on the cooling load and ambient dry-bulb temperature profiles by using the proposed approach. The integrated simulation tool is validated by using a wide range of experimentally collected data from the chiller in operation. Simulation results are provided to show possibility of significant energy savings and comfort enhancement using the proposed strategy.

Technical and economic analysis of a solar-assisted air-conditioning system

This paper presents analysis aiming at assessing the feasibility and economic performance of a solar-assisted air-conditioning system for a middle class house under the climatic conditions of Tunis City. Two alternative cooling modes for the single effect water-lithium bromide absorption machine are considered, a water-cooling in association with a cooling tower and air-cooling. The numerical simulations show that for the water-cooling case, a solar system with 18 m2 of evacuated tube collector area and a thermal storage tank of 1.3 m3 maintained at a temperature of 95°C can cover 50% of the energy needs of the machine. The corresponding optimal results for the air-cooled chiller are: energy needs coverage 40%, hot water tank capacity 1 m3 at 110°C, collector area 14 m2. The annually total costs for the case of water-cooling and air-cooling are respectively 3.52 €/kWh and 2.51 €/kWh.

An analysis on the energy efficiency of air-cooled chillers with water mist system

This paper presents the effect of operating water mist system to enhance the energy efficiency of air-cooled chillers under various operating conditions. A thermodynamic model for an air-cooled chiller with twin refrigeration circuits coupled with a water mist system was developed and validated by measured operating data. The validated model was used to investigate how the thermal properties of the entering condenser air and the performance of the air-cooled chiller varied under different operating schemes with water mist pre-cooling. Under the conventional head pressure control (HPC) with a designed water mist generation rate, the coefficient of performance (COP) increased by up to 21.3%. The chiller performance could be improved further under variable condensing temperature control (CTC). Under CTC, chiller COP was increased up to 51.5% with optimal water mist generation rate. The potential energy savings of these chillers serving a representative office building in subtropical climate was evaluated, and CTC coupling with optimal water mist generation rate would reduce the annual total electricity consumption for cooling by 14.1%. The water consumption of the water mist system was comparatively small, comparing with the total water losses for an open-loop cooling tower system with equivalent heat rejection capacity.

Improved energy performance of air-cooled water chillers with innovative condenser coil configurations – Part ΙΙ: Experimental validation

The second part of this study (Part II) used full-scale experimentation to investigate the manner in which how two condenser coil configurations (VRC and CRC) influence the performance (energy efficiency) of the system and individual components of air-cooled chillers. The results showed that, for both DX and FL evaporator configurations, the chillers with VRC condensers had a greater cooling capacity (an increase of 4.5% and 4.0%, respectively), refrigeration capacity (an increase of 6.5% and 5.6%, respectively), and COP (an increase of 7.3% and 6.7%, respectively), compared to chillers using CRC condensers. This demonstrates that VRC-designed condensers can effectively improve heat transfer performance and enhance the energy efficiency of refrigeration systems without increasing material costs or the number of tubes and fans. The results of this study can be used as reference data for future research on improving the system performance of air-cooled water chillers.